1. Field of the Invention
The present invention generally relates to a method of determining optical properties in a projection exposure system and a projection exposure system comprising a wavefront detection system. In particular, the present invention relates to the determination of optical parameters from one or more wavefronts in a projection exposure system wherein light of more than one wavelength is used for exposure.
2. Brief Description of Related Art
Lithographic processes are commonly used in the manufacture of semiconductor elements, such as integrated circuits (ICs), LSIs, liquid crystal elements, micropatterned members and micromechanical components.
A projection exposure apparatus used for photolithography generally comprises an illumination optical system with a light source and a projection optical system. Light from the illumination optical system illuminates a reticle (mask) having a given pattern and the projection optical system transfers an image of the reticle pattern onto a photo-sensitive substrate. The image of the reticle pattern may also be reduced in size by the projection optical system so as to produce a smaller image of the reticle pattern on the substrate.
The trend towards ever more sophisticated semiconductor devices requires semiconductor elements of smaller size and higher complexity which, in turn, makes higher demands on the optical performance achievable with projection exposure systems. In particular, the image of the reticle pattern needs to be very accurately projected onto the substrate. Therefore, for instance, aberrations of the wavefronts of the light transferring the image of the reticle pattern onto the substrate need be to reduced to the greatest extent possible.
Various methods and systems are known which allow to alter the optical performance of the projection exposure systems after assembly of the various components comprised in the projection exposure system. For instance, one or more lenses can be tilted or moved along an optical axis of the system to reduce certain types of aberrations, such as astigmatism. After assembly and optimisation, the optical performance of a projection exposure system may undergo a change induced by a variation in atmospheric conditions of the environment the projection exposure system is exposed to, for example. The optical performance of the projection exposure system may also change over time.
In order to be able to optimise the optical performance of a projection exposure system and make adjustment(s) thereto, optical properties of the projection optical system need to be accurately determined.
Various methods of determining optical properties in a projection exposure system and detection systems for detecting these properties are known. Amongst those, the most preferable are those which allow for in-situ measurements of the optical properties in the projection exposure system and do not require significant alteration of the projection exposure system. In-situ measurements thus increase ease of use and time-efficiency of the projection exposure system.
Among the commonly employed wavefront detecting techniques is the Shack-Hartmann-technique. This technique involves splitting a wavefront in a pupil plane of the projection optical system into a plurality of portions and measuring a gradient of each split wavefront portion. Thus, an aberration of the split portion and an aberration of the whole wavefront can be determined. An example of a wavefront detecting method using the Shack-Hartmann-technique is described in US 2002/0159048 A1, for instance, the entire content of which is incorporated by reference herein. Alternatively, the classical Hartmann test technique may be used for measuring wavefront profiles.
Other wavefront detection methods known in the art are based on interferometric techniques. Point diffraction interferometry, Twyman-Green interferometry, Fizeau interferometry, or shearing interferometry methods may be used, for instance.
U.S. Pat. No. 5,828,455, the entire content of which is included by reference herein, describes a method and apparatus for analysing a wavefront at a multiplicity of field points over an entire lens train. The method is based on the finding that upon irradiating a reticle, spots are created at an image plane of the projection optical system which are deviated from their diffraction-limited positions, which deviation can be measured. The method includes using an aperture plate consisting of a multiplicity of openings with each opening being centred underneath a neighbourhood of points accepted into an entrance pupil of an imaging objective. Points traversing through all openings will produce a number of spot arrays in a substrate plane. The totality of all the arrays of spots whose centroids can be measured and reconstructed yields an aberrated wavefront at a number of discrete field points. Thus, an in-situ interferometric analysis of a wavefront having traversed the projection optical system in a projection exposure system is provided.
In US 2002/0001088 A1 a method and apparatus for detecting wavefronts by shearing interferometry are disclosed, the entire content of which is incorporated by reference herein.
U.S. Pat. No. 6,344,898 discloses a point diffraction interferometer for measuring a test surface by detecting the state of interference fringes generated by interference of a reference light beam that interacts with the test surface. The reference and measurement beams are produced by a point light source. In WO 02/42728, a method and apparatus for measuring an aberration of a projection optical system are described which are based on point diffraction interferometry. The entire contents of both referenced documents are incorporated herein by reference.
Wavefront detection techniques for in-situ determination of optical properties require a wavefront source and, hence, a light source. Generally, for reasons of ease of use, a beam generated by the light source comprised in the projection exposure system will be used both in an exposure mode for exposing a substrate and a measurement mode for detecting a wavefront and determining optical properties in the projection optical system.
With projection exposure systems being developed for ever smaller wavelengths, among the light sources currently used or being envisaged for use are excimer lasers such as ArF lasers (193 nm) and KrF lasers (248 nm) as well as F2-lasers (157 nm), which emit light in the ultraviolet range. In addition, the development goes towards use of light sources emitting light in the extreme ultraviolet end (EUV) of the spectrum, such as plasma sources.
The range of wavelengths of light emitted by the various light sources differs largely. While lasers are usually adapted to emit substantially monochromatic light, i.e. emit light of one wavelength having a small spectral bandwidth, plasma sources used in EUV projection exposure systems tend to emit a broad band of wavelengths. In view of the requirements of the projection exposure system in terms of accurate projection of an image of the reticle pattern onto the substrate, the light source will have to fulfil certain specifications in terms of feasible spectral bandwidths and wavelengths.
However, whilst the transition to smaller wavelengths has been well accommodated in terms of a quality achievable in an exposure mode, results from wavefront detection techniques measuring at the wavelength of exposure have been found to be unsatisfactory.